RepRap X2 is a dual extruding 3D printer based on Prusa's Mendel. A typical Prusa's Mendel with RAMPS electronics can be
converted to RepRap X2 with a few hardware changes. The firmware and the host software changes are necessary in order to
utilize the dual extrusion capabilities of the RepRap X2 printer.

The information published here outlines the hardware changes as well as where to get and how to use the firmware and the host software
for RepRap X2 3D Printers.

The hardware modifications published here are for RepRap X2 v2.0. They will work for the current (year 2011) Prusa's Mendel (regardless
metric or SAE version) w/ Greg Frost's extruder v3 (variations should work too) and Makergear hotends.
The firmware is based on Marlin v1 and the host software is X2SW software bundle
combining Printrun, Skeinforge, Slic3r and X2 Profiles manager into a single software product.
The X2 firmware was so far tested with RAMPS v1.4 electronics, but any electronics package supported by the firmware and capable
of driving one more stepper motor and hotend (heater and temperature sensor) is likely to be compatible.

You can also find the older RepRap X2 description and the downloads on Thingiverse: Thing 16523

Why 2 extruders? There are several reasons for doing that:

printing objects of an arbitrary shape;

printing multi-color objects;

spending less time switching extruders or reloading the filament.

The most interesting are the first 2 options. With the introduction of the water soluable (PVA) filament,
the 2 extruder printers can potentially be used to print any shapes by enclosing the object volume printed in
normal plastic in the volume of the water soluable PVA. The model is made out of 2 objects, one is for printing the "enclosure" in PVA,
another one is the complex shape object itself for printing in normal plastic. The objects are sliced with the same layer height and
the resulting g-code is mixed by a script to print "combined" layers. So far I've successfully used the technique for printing in
different color plastics. Using PVA for supporting PLA oject appears to be promising (I've had good adhesion) but PVA turned out to be a
very capricious material to work with and I haven't had a success yet with any large complex objects

Alternatively, it is possible to set up the slicing software to just generate support material and include g-code for switching extruders
when switching between support and the object itself. The latter method isn't the best solution for the 2 extruder system since the slicing
software (I mean Skeinforge here) was designed for single extruder printers. It generates support using the same slicing parameters and
it is normally a "grid" that has gaps causing deterioration of the quality of the supported layers.

The multicolor printig can be done using the same object "splitting" technique. First, the object volumes that have to be printed in different
colors are separated, then sliced individually and the sliced layers are merged together. Here are a couple of links to objects I've designed and
printed this way: Japanese Puzzle BoxCalibration piece for 2 extruder/color printing

The potential alternative method is to modify the slicing software to change the color only for printing the perimeter and grab the color information
from the 3D object textures. If done, this can potentially be used to mix colors when more than 2 extruders are available (i.e. real color 3D printing).
However this method requires substantially more changes/work to get any useful results.